Piezoelectric thin films are processed into various piezoelectric devices depending on their intended purpose, and are widely used as electronic components such as an actuator for deforming a device by applying a voltage, or a sensor for generating a voltage by deforming the device. For example, Japanese Laid-open Patent No. 2002-279742 discloses a technology of fine control of a head position in a magnetic disk by using a piezoelectric device. The piezoelectric device is used because the recording density of the magnetic disk is increased, the area of a recording region of one bit is decreased, and it is difficult to obtain a sufficient precision by positioning of the head using only the conventional voice coil motor. Accordingly, in addition to positioning by utilizing the voice coil motor, it has been considered to compose a two-stage actuator for positioning at high precision in a very small region by a piezoelectric device. A piezoelectric device unit used for this purpose is composed of a pair of piezoelectric devices, which are disposed so that one of the piezoelectric devices is contracted when the other of the piezoelectric devices is expanded, and therefore, the head provided at the leading end can be moved finely at a high precision on the disk surface.
Such a piezoelectric device is generally manufactured in the following manner. A substrate for the piezoelectric device is, for example, a magnesium oxide single crystal substrate (MgO substrate). On this MgO substrate, a platinum film (Pt film) oriented by (100) is formed. On this Pt film, a lead zirconate titanate (PZT) thin film oriented by (001) is formed. Further, an electrode thin film is formed on the PZT thin film, and these thin films are processed into a specified shape by photolithography and etching. Finally, by removing the MgO substrate by etching or other process, a piezoelectric device is fabricated.
For forming a PZT thin film, a sputtering method is generally employed, and its substrate temperature is 550 to 650° C. By sputtering at such a high temperature, lead (Pb) is evaporated from the PZT thin film in the sputtering process, and the final fabricated PZT thin film deviates from the stoichiometric composition due to a decrease of Pb composition. To obtain the PZT thin film of stoichiometric composition, Takayama et al. attempted to compensate for the Pb component in the PZT thin film by composing the target for sputtering to contain Pb by about 20% in excess, and forming the film by using this target (J. Appl. Phys. 65 (4), 1666, 1989).
However, to obtain the PZT thin film of stoichiometric composition by using the target containing Pb in excess, it is necessary to add oxygen gas to an inert gas as a discharge gas for sputtering, and form the film by sputtering in a condition of relatively high pressure. Under such a condition, the film forming speed cannot be increased. When used as a piezoelectric device, the PZT thin film is required to form about 1 μm to 10 μm in thickness, and the mass productivity is extremely reduced at a slow film forming speed.
Japanese Laid-open Patent No. H6-49638 discloses a technology of sputtering at a relatively low discharge gas pressure in a vacuum apparatus which increases the film forming speed when forming a PZT thin film for use in a semiconductor memory. When the discharge gas pressure is low, the Pb component in the formed film is likely to decrease, and therefore, to fabricate a PZT thin film of stoichiometric composition, a target of excessive Pb composition is used depending on the discharge gas pressure.
On the other hand, Japanese Patent Publication No. 3341357 discloses a technology of increasing the piezoelectric constant d31 by utilizing excessive Pb in the thin film fabricated for improving the piezoelectric characteristics of the PZT thin film more than in the stoichiometric composition, and forming a rhombohedral crystal structure.
In the first prior art example and the second prior art example, however, both methods aim to fabricate a PZT thin film of stoichiometric composition, and it is necessary to form the film by sputtering in a discharge gas of relatively high oxygen partial pressure. In the PZT thin film formed at such high oxygen partial pressure, the piezoelectric constant d31 is generally small, and the film forming speed in sputtering cannot be increased. Hence, good piezoelectric characteristics are not obtained, and the mass producibility is not improved.
In the third prior art example, the Pb amount in the PZT thin film is in excess of the summed amount of titanium (Ti) and zirconium (Zr), but the ratio of oxygen (O) and Pb in the PZT thin film is increased at a same rate, and oxygen deficiency does not take place. Accordingly, when forming the film, oxygen must be added and sputtering must be done in the condition of high discharge gas pressure, and therefore, the film forming speed is not increased. Hence, mass productivity is not improved.